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Sunn 200S


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I think I may have found the problem with my Sunn 200S.

Some weeks ago, after band practice, I flipped the Standby switch, and the fuse blew. Replaced the fuse, and it blew again.

Finally got around to doing some troubleshooting, but everything seemed good, until I turned it on, and it blew another fuse. But I think I have it now: bad recitfier diode.

"Bad diode?" you ask. Yes. I did a mod some years ago, replacing the rectifier tube and power supply caps with a SDS Mk3 capacitor board. Gives the amp a bit more power, eliminates sag (good for bass, not so good for guitar, IMO), and results in a quieter, cleaner amp. But not so good if a diode goes out. I think what's happening is that a diode has shorted, and shorts out the main transformer secondary winding, drawing too much current and blowing the fuse.

We'll see. New diodes arrive tomorrow.

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Went well.

I received the bag o' diodes on Wednesday (I don't expect I'll ever need to buy any more!). Thursday, while my lovely wife was out, I pulled the old diodes and tested them: one good, one shorted. Replaced both diodes, because why not? Fired up the amp in standby: no blown fuse!

Replaced the tubes, and turned it on again: so far, so good.

A quick note here: it's often a bad idea to power up a tube amp without the tubes in place. The reason is that the tubes bring down the voltage by drawing current. Without them in place, the voltage can be too high for the filter capacitors. I admit that I've done it before, and got away with it, but I don't recomment it. In the case of the 200S, and all Sunn amps of that vintage, voltage is not applied to the caps when the amp is in standby.

Hit the standby switch, and was treated to a dazzling light show from one of the power tubes! Killed the power, but blew another fuse. Replaced the fuse and switched the tubes around, figuring that, if it's a problem with the amp, I'd get the same result with a different tube. If the problem was the tube, then the problem should move with it. Instead, everything appeared to be fine. At this point, I'm thinking that the tube was bad, and burned out when I powered it up.

Gig tomorrow. After, I'll replace the power tubes and test the amp on a bass speaker.

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you might want to get a read on the input voltage on that tube socket. It might be over spec, just enough to blow an older tube.

Agreed that powering up a tube amp absent the tubes is a potential death sentence for the filter caps...interesting that in standby the Sunn doesn't light them up. Also powering up without a load on the OT can be bad for the amp as well.

I have not worked on a Sunn tube amp, but when you described the mod you had done, and the fuse blowing repeatedly, my first inclination was fried diode...:thu:

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